The present invention relates to an instrument for spectral measurement in the bloodstream, having a light-guide probe and a diode line spectrometer.
A photometric measurement in the bloodstream is of importance, for instance, for the continuous measurement of the oxygen content of blood. This is of particular importance for anesthesia since an under-supply of oxygen during anesthesia may lead to severe and irreparable damage to the patient. The only method of measurement which is widely used up to the present time in routine procedure is a polarographic method (the "Oxymeter" of Drager AG and Hellige GmbH) which has been in use since 1975 and is based on a proposal made by Huck and Lubbers (Arch. Gynakol. 207, 443, 1969). The method has the disadvantages of an "inflow time" of 15 to 20 minutes, a system-caused delay time of about 15 seconds, the necessity of calibrating before each measurement, and a drift of 10 to 15% within a few hours also inherent in the system. Furthermore, it is necessary to occasionally carry out arterial blood gas analyses for functional verification of the electrode.
From West German unexamined patent application (offenlegungsschrift) No. 27 26 606 there is known a spectral photometer for measurement on tissue surfaces in which the spectrum is made visible on an oscilloscope. The photometer is provided with a photocell array; the measurement head is connected to the apparatus by a light guide.
In unexamined application for a patent of addition (zusatz-offenlegungsschrift) No. 28 15 074 to the aforementioned application, the display screen of the oscilloscope has marks located at the characteristic points of the hemoglobin spectrum to facilitate evaluation.
It is known, for instance from Documenta Geigy, Scientific Tables, 6th Edition, 1960, that the absorption maxima of the spectra of hemoglobin, the reduced form, and of oxyhemoglobin, the oxidized form, shift in characteristic manner: the long-wave absorption maximum shifts from 560 to 577 nm and the short-wave maximum from 430 to 412 nm. The distance between the two maxima therefore changes by 35 nm as a function of the oxygen content, regardless of the absolute value of the absorption. This lack of dependence on the absolute value of the absorption is the most important prerequisite for the possibility of effecting an exact determination of oxygen content. It is known and obvious that the oxygen content "in vivo" can be determined only with the exclusion of atmospheric oxygen. It is furthermore known that the number of erythrocytes, the carrier of the hemoglobin, is about 5 million per cubic millimeter (40 to 45% of the volume of the blood), with an average of 7.5 .mu.m. This means, that as a result of the high scattering coefficients photometric transmittance measurement would be impossible. On the other hand, however, the opacity is still so great that true reflectance measurement is also not possible.
From an article by Curtis C. Johnson in the Journal of the Association for the Advancement of Medical Instrumention, 5, 77 (1971), it is known to effect a reflection measurement in vivo with a light-guide measurement head which is introduced into a heart catheter. The measurement is effected with two wavelengths; two LEDs are used as sources of light and a silicon photodiode is used as receiver. The measurement within the heart is necessary due to the large diameter of the light-guide. It is self-evident that such a measurement enters into question only in exceptional cases and is not intended for routine use in normal operations.
From an energy standpoint, the device described by U. Tutschke in Biomedizinische Technik 21, 279 (1976) is much more favorable. An HeNe laser and a pulse dyestuff laser are used for producing the measurement light with two different wavelengths. The much greater luminous intensities than with traditional sources of light result in corresponding return scatter signals so that as compared with other methods it is possible to measure with a more favorable signal-to-noise ratio and/or to use light guides of smaller diameter which make intravasal measurement possible. For routine medical use a measurmeent with two lasers is, however, too expensive, so that such an instrument does not satisfy practical requirements.
The measurement at two wavelengths cannot directly determine the spectral displacement of the absorption maxima but merely the change in intensity of a characteristic wavelength with respect to an isobestic point. For quantitative determinations it is more favorable to have the entire spectral course available in the relevant region and thus improve the accuracy. Furthermore, an instrument which measures the entire course of the specrtrum can be universally used.
The object of the present invention is therefore to provide an instrument suitable for routine medical use which makes it possible to measure the scattered light spectrum of blood within a peripheral vein or artery without interfering with the free circulation of the blood through the blood vessel.